When I open this S/O template report in the report designer I noticed that the 'VTA' is just text. So I changed this to 'VAT'.

VTA : [[ (o.partner_id.vat) or removeParentNode('para') ]]
changed to

VAT:[[ (o.partner_id.vat) or removeParentNode('para') ]]

For English customers this is perfect, but for my dutch customers it isn't, because now the translation doesn't work.
The VAT term should be translated to 'BTW' but it doesn't, the translation only works when I use the term 'VTA' in the template.